For structures or devices manufactured in a layer-by-layer approach, like for example semiconductor devices on a wafer, correct positioning of patterns relative to each other, in one and the same layer or in different layers, is of crucial importance for the function of the device. For example, correct relative positioning of such patterns is important to achieve a desired electrical contact between such patterns, or to avoid an undesired electrical contact between such patterns. Inadequate control of the positioning of patterns therefore can lead to high losses during production of the devices, i.e. to low production yield.
Prior art achieves control of the positioning of patterns by what is known as overlay (OVL) measurements. OVL metrology based on optical images and/or on optical scatterometry is currently typically used for controlling the correct positioning of semiconductor devices on substrates. These optical measurements may be augmented by other methods, like Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM), used to validate or even calibrate/offset the results of the optical measurements. OVL measurements reveal the relative position of patterns placed on a substrate. Optical OVL measurements are currently possible only for specially designed metrology targets, i.e. patterns specifically provided on the substrate, or layers thereon, for the purpose of such measurements; optical OVL measurements on functional patterns, i.e. on patterns which are provided to perform some function in the semiconductor device manufactured, in most cases are not possible. From the principle of OVL measurements, measuring the relative position of patterns placed on the substrate, it is clear that OVL information can only be used as a feedback for the manufacturing process, i.e. the OVL information is only obtained after the layers on which the OVL measurement is performed have been placed on the substrate and patterned. This information can then be used in the creation of further patterns, and in particular if patterns corresponding to the patterns measured are created on a subsequent substrate, i.e. on a substrate on which the creation of these patterns is performed after the OVL measurement which provided the information.
Thus, based on OVL information, i.e. information obtained from OVL measurements, it is not possible to predict the position of patterns yet to be placed on the substrate, so no feed forward of OVL information is possible. The reason is, as should be clear from the above, that OVL measurements only reveal the relative position of patterns. If, for example, two layers, a first layer and a second layer, have already been placed on the substrate, and a third layer is to be placed on the substrate, then, in order to achieve correct positioning of the patterns in the third layer with respect to the second layer, the pattern placement in the second layer has to be known before forming the patterns in the third layer. OVL measurements, however, in this case provide only the relative positions between patterns of the first layer and of the second layer. Separate information on the placement of the patterns in the second layer cannot be extracted from these relative positions. The fact that only specially designed metrology targets can be measured, while it is the functional patterns where the correct placement is crucial, is a further disadvantage of prior art. As these metrology targets are typically larger than 10 μm by 10 μm, it is not possible to place a large number of such metrology targets in the vicinity of functional patterns.